System design recommendations
Models: NTZ048 - NTZ068 - NTZ096 - NTZ108 - NTZ136
Models: NTZ215 - NTZ271
Suction accumulator
Liquid line solenoid valve
& pump-down
Crankcase heater
A suction accumulator offers considerable
protection against refrigerant floodback both at
start-up and during operation or after the defrost
operation. This device also helps protect against
off-cycle migration by providing additional
internal free volume to the low pressure side of
the system.
The suction accumulator must be selected in
accordance with the accumulator manufacturer
recommendations. As a general rule, Danfoss
Commercial Compressors recommends to
size the accumulator for at least 50% of the
total system charge. Tests however must be
conducted to determine the optimal size.
In refrigeration applications, the Liquid Line
Solenoid Valve (LLSV) is highly recommended.
During the off-cycle, the LLSV isolates the liquid
charge in the condensor side, thus preventing
refrigerant transfer or excessive migration of
refrigerant into the compressor. Furthermore,
when using LLSV in conjunction with the pump-
down cycle (especially in low-temperature
applications), the quantity of refrigerant in the
low-pressure side of the system will be reduced.
A pump-down cycle design is required when
evaporators are fitted with electric defrost
heaters.
R
A crankcase heater protects against the
off-cycle migration of refrigerant and proves
effective if oil temperature is maintained 10
K above the saturated LP temperature of the
refrigerant. Tests must thereby be conducted to
ensure that the appropriate oil temperature is
maintained under all ambient conditions. A PTC
crankcase heater is required with all Maneurop®‚
NTZ compressors. PTC crankcase heaters are
self-regulating.
Under extreme conditions such as low ambient
temperature at –15°C or lower a belt type
crankcase heater could be used in addition to
the PTC heater, although this is not a preferred
solution for 1 and 2 cylinder compressors. The
belt crankcase heater must be positioned on the
compressor shell as close as possible to the oil
sump to ensure good heat transfer to the oil.
The below illustrated mounting positions are
recommended:
Belt crankcase heaters are not self-regulating.
Control must be applied to energise the belt
heater once the compressor has been stopped
and then to de-energise it while the compressor
is running. The belt heater must be energised
12 hours before restarting the compressor
following an extended down period.
If the crankcase heater is not able to maintain the
oil temperature at 10 K above the saturated LP
temperature of the refrigerant during off cycles
or if repetitive floodback is present a the Liquid
Line Solenoid Valve (LLSV) + pump-down cycle is
required, possibly in conjunction with a suction
accumulator.
R
Crankcase heater or PTC crankcase heater
always has to be used when refrigerant R454C/
R455A is used.
AB245486497018en-000601
24
Application Guidelines
